Find catalog duplicates and fold a group onto one copy in one transaction

The library holds the same RAW in several folders: a dated folder, a
bck/ beside it, a renamed Darktable export tree. dr_catalog::duplicates
is the catalog half of consolidating them (#67).

candidates() is one grouped query over root, camera, capture instant and
size, joined back for the rows; count() is the same grouping under
COUNT. On a copy of the reference catalog (23,582 images) both take
10-35 ms and find 1,836 groups holding 3,379 spare copies.

survivor() prefers a copy outside a backup-looking folder, then one
still named the way the camera named it, then the oldest, then the
lowest id.

consolidate() re-checks the plan against the catalog, merges the copies'
judgements onto the survivor (highest rating, keywords unioned,
collections unioned with the survivor keeping its place, a flag or label
the copies agree on, faces via faces::carry_onto_copy) and records the
copies as trashed, all in one transaction, so a failure part way leaves
the group untouched. preview() runs the same code and rolls it back.

Sameness probes are kept in dedup_probes, created on first use rather
than by a migration: a schema bump would make older builds refuse this
catalog's snapshot at sync. trash::record_trashed_within lets the trash
write share the merge's transaction.

core/dr-catalog/src/lib.rs

@@ -40,6 +40,7 @@ pub mod bursts;
 pub mod cache;
 pub mod collections;
 pub mod dedup;
+pub mod duplicates;
 pub mod error;
 pub mod face_shard;
 pub mod faces;